。
People who get taken for a ride really do give off a 'mark' aura, I suppose.
Literal
Duck-[target] made-into person [explanation-というのは] as-expected duck [possessive-の] aura [object-を] emitting [explanation-ということ] [probably-でしょう].
鴨 ('duck') is slang for an easy mark or sucker — from the proverb 鴨が葱を背負って来る ('a duck comes carrying green onions on its back'), meaning someone who makes it too easy to take advantage of. The sentence uses 鴨 twice: once for being made into a mark (鴨にされる) and once metaphorically for the 'aura' such people emit. やはり ('as expected') implies the speaker has confirmed a suspicion.